As I explained last month, the BLS releases six unemployment measures. There’s U3, the number that shows up in all the news article, which counts people who don’t have jobs, but have looked for one in the past four weeks, but U1, U2, U4, U5 and U6 exist as well. U1 and U2 are usually lower than U3, and measure the percentage of people who have been unemployed for 15 weeks or longer and the percentage who have lost jobs or done temporary work in the period in question, respectively.

U4, U5 and U6 are usually higher than U3. Each of these categories includes everyone in all the lower categories: all people in U3 are in U4, all people in U4 are in U5, and all people in U5 are in U6. U4 adds people who have stopped looking for work because they’ve concluded none is available. U5 adds people who would like to work but for whatever reason have not looked for work recently. U6 adds the underemployed, or part-time workers who want to be working full-time but cannot for whatever reason.

The six measures are pretty well coordinated, and all but U6 ticked down in September, suggesting again that the drop reflects real improvement.